There are many examples where it is necessary to know accurately the moisture content in a medium. This is particularly relevant in relation to soils where the moisture content is important for agricultural purposes. The ability to determine accurately the moisture content in soils will enable determination of watering rates, and when such watering is required, and is also useful in relation to automatic control of various irrigation systems.
One well known method of determining moisture content of mediums such as soils is the use of electrical conductivity probes. However the sensitivity of such conductivity probes varies greatly with the amount of moisture present, and such probes do not accurately sense moisture content in soils when containing only a small degree of moisture. Also, reading variation is sensitive to the (dry) resistivity of the medium being measured, and variations in the degree of dissolved salts. Other conductivity methods, such as using a porous material in the soil to absorb an equilibrium of moisture suffer from similar problems, and are prone to clogging from fine particulate matter, or sediment from dissolved salts etcetera. Further, it is difficult to use conductivity probes or the like to continuously monitor moisture on a regular, unattached basis, due to variations in reading due to dissolved salts or contamination of the porous material other methods of measuring soil moisture, such as the neutron probe (atomic bombardment of the hydrogen atom), or tension meters (water surface tension suction), by their nature of cost are regulated to the manual measurement, laboratory instrument' class.
Therefore, it is an object of this invention to provide a simple means in determining moisture content in a medium, wherein that means will provide more accurate readings over a wide range of moisture content and soil type, than one available from conductivity probes, while enabling "continuous" regular readings to be obtained automatically, without degradation of the measurement probe.
Soil varies greatly in composition, and can for example comprise coral sand (coralite, comprising absorptive calcium carbonate particles), silicon sand (comprising non-absorptive grains of silicon di-oxide), ironstone particles or pebbles (which are conductive of electricity and heat), as well as the more common clays and loams. A series of tests has shown that utilisation of the specific heat value of the amount of moisture content provides a very much more accurate and sensitive basis for estimating moisture content than previously used variation of electrical conductivity, probably due largely to negligible effect of variation in specific heat due to variation of salt content of water, compared to the very wide differences in electrical resistivity of the soil components.